1. Field of the Invention
This invention relates to synthetic resin coatings capable of dissipating surface electrostatic charge and more particularly to ultraviolet radiation-curable compositions for preparing coatings having high abrasion resistance in conjunction with permanent anti-static properties.
2. Brief Description of the Prior Art
Coatable compositions curable by ultraviolet radiation are well known in modern coating technology. Such coatings are convenient to apply and are environmentally advantageous because they typically contain no volatile organic solvents. However, such coatings, generally formed from synthetic resins, are often lacking in the hardness needed to prevent scratching of the surface in use, with accompanying dulling of the original glossy finish. Such scratching is especially undesirable when the coating is applied to a substrate that is intended to be transparent, e.g., glazing, cathode ray tube protective screens, enclosures for electronic instruments or the like. The accumulation of surface scratches on such articles causes the article to become hazy and decreases the optical transmission properties of the article. Such coatings also may have very high surface resistivity, which makes for very slow dissipation of static electric charge that may accumulate on the surface of the coated article. Hard, abrasion-resistant coatings have been prepared by polymerizing monomers having more than two polymerizable groups per monomer molecule to form highly cross-linked coated layers. However, such hard coatings are especially prone to have high surface resistivity.
Auxiliary anti-static coatings, sprayed onto the surface, have been used to alleviate the problem of retained static charge. However, such sprayed coatings are not permanent, but must be renewed from time to time. Accordingly, attempts have been made to provide permanent anti-static properties by increasing the intrinsic surface conductivity, i.e., decreasing the surface resistivity, by incorporating electrically conductive materials into the coatings. For example, U.S. Pat. No. 5,721,299, to Angelopoulos et al., discloses electrically conductive, scratch-resistant polymers incorporating conductive polymers. However, incorporation of other materials in significant amounts can tend to reduce the hardness of the coating and thereby its resistance to scratching.
Accordingly, a need has continued to exist for a hard, abrasion-resistant coating that has good static electricity dissipation properties, especially for use in coating transparent substrates.